OX40 (CD134) is a member of the TNFR-superfamily of receptors. Predominantly expressed on activated T cells, OX40 is activated by its cognate ligand OX40L (CD134L) and functions as a T cell co-stimulatory molecule. OX40-OX40L interactions have been proposed as a potential therapeutic target for treating autoimmunity. OX40 can interact with TRAF2, TRAF3, and TRAF5. In addition to a membrane bound form, OX40 also exists as a soluble form, released by activated lymphocytes.